1. Field of the Invention
The invention relates to a light source device which is used, for example, as a light source for a projector, and in which a discharge lamp with high radiance (HID lamp), such as a high pressure mercury discharge lamp, a metal halide lamp or the like, is used. The invention relates especially to the starting properties of such a device.
2. Description of Related Art
In a light source device for an optical device, such as a liquid crystal projector, a DLP® projector or the like, a discharge lamp with high radiance which is used as a light source, such as a high pressure mercury discharge lamp, a metal halide lamp or the like, and a reflector with a reflection surface which focuses the radiant light from this discharge lamp and reflects it in the direction toward the front opening are combined with one another and used.
In the above described discharge lamp, it is generally necessary when starting to apply a high voltage in pulse form between the electrodes for the main discharge or between the electrode for the main discharge and the inside of the discharge vessel, to produce an insulation breakdown in the discharge medium within the discharge vessel and to induce a glow discharge or an arc discharge, the plasma electrons which are produced in doing so acting as triggers.
The voltage which is necessary for an insulation breakdown when starting a discharge lamp is generally a few kilovolts in the case in which this discharge lamp is in a temperature state which is roughly similar to room temperature. The voltage necessary for an insulation breakdown in a restart changes, however, depending on the time which has passed since turning off after completion of prior operation, i.e., depending on the temperature of the discharge space. It can be imagined that the reason for formation of such a change lies in the following.
According to the reduction of the temperature of the discharge space after the lamp is turned off, the part of the discharge medium which was gaseous, such as mercury, a halogen and the like, begins to condense. As a result, the composition of the gaseous portion of the discharge space changes, by which the voltage which is necessary for the insulation breakdown changes.
In the case, for example, of a discharge lamp in which mercury and a halogen, such as bromine or the like, and a rare gas, such as argon or the like, are used as the discharge medium, in the case in which, for example, at least 0.15 mg of mercury per cubic millimeter volume of the discharge space (Zd) is contained, the voltage which is necessary for the insulation breakdown after turning off the discharge lamp due to the presence of residual plasma is very low. It does increase rapidly thereafter, but soon begins to drop (roughly 2 minutes under the condition of natural cooling under which the discharge lamp is not subject to compressed air cooling). However, there are cases in which, for example, afterwards, in a restart of roughly five minutes operation after turning off, until finally the temperature of the discharge space drops to roughly 100° C. or less, the breakdown voltage does not stabilize and in which, at the applied high voltage, an insulation breakdown does not occur.
In order to carry out a restart (hot restart) as soon as possible after the lamp has been turned off and to further increase the probability of operation, it is simply enough if the absolute value of the high voltage which is to be applied is fixed to be high. In the case of this measure, there are cases in which different disadvantages occur, such as the formation of an unintentional insulation breakdown by the applied voltage, i.e., the formation of an insulation breakdown of the coating of the insulated cable or the formation of a dangerous phenomenon, such as a creeping discharge or the like, on the connector or the connecting terminal and a malfunction of the electronic circuit of the projector device main part which is caused by noise when the high voltage is applied, and similar disadvantages. If there is a greater spatial distance to increase the insulating property, or if the cable diameter is increased to prevent noise, in order to avoid these disadvantageous phenomena, sufficient space is required for installation in the projector device. Therefore, this measure is not desirable.
With respect to improving the starting property of a discharge lamp, a technology was proposed in which light with short wavelengths, such as UV radiation or the like, is used to accelerate photoemission by the photoelectric effect on the material within the discharge vessel and the ionization of the discharge medium, and to reduce the absolute value of the high voltage which is to be applied when starting. For example, U.S. Pat. No. 5,323,091 (parallel disclosure of the international patent application: WO-A-00/77826) discloses a technology in which bubbles are formed in the discharge vessel of the discharge lamp itself and a secondary discharge chamber is formed which emits UV radiation.
Furthermore, for example, U.S. Pat. No. 6,268,698 (parallel Japanese patent application: JP-OS 2000-173549) proposes a discharge lamp in which on the end face with a hermetic seal arrangement of the discharge lamp an auxiliary UV light source which discharges into open space is installed in one piece. However, in the respective prior art production costs are high because production of the discharge lamp is difficult or otherwise reliability is lacking with respect to the pressure tightness of the discharge lamp.
As generic technology of the invention, the assignee of the present application has devised an invention which does not constitute prior art and which is described in Japanese patent application 2002-2317. The feature of this application lies in eliminating the disadvantages in the prior art and arranging an auxiliary discharge vessel with a main discharge vessel asymmetricly and adjacent to at least one of the sides of the electrode sealing part of the main discharge vessel which closes the main discharge. Here, the overall length of the auxiliary discharge vessel is adjusted to the dimensions of the above described electrode sealing part, and furthermore, the outside diameter of the auxiliary discharge vessel is controlled in such a way that the radiant light flux from the main discharge vessel is not shielded.
Recently, there has also been a great demand for reducing the size and weight of a liquid crystal projector device. Accordingly, it is desired more and more often that the light source device be made smaller. For this purpose, a shortening and a reduction of the overall length of the main discharge vessel are even more required. In order to meet this demand, it is necessary to make the auxiliary discharge vessel even smaller. In the technology which has already proposed by the assignee of the present application, however, according to the reduction in the size of the auxiliary discharge vessel the difficulty of its manufacture becomes greater, by which a reduction of the quality and a cost increase presumably occur. To avoid these problems, a light source device is desired with an arrangement in which the dimensions of the auxiliary discharge vessel need not be adjusted.
In the case in which the above described auxiliary discharge vessel is located directly tightly adjoining the main discharge vessel, this auxiliary discharge vessel is more often exposed to the heat from the main discharge vessel, by which the gas pressure within the auxiliary discharge vessel increases, and thus, the breakdown voltage increases. Starting the discharge within this auxiliary discharge vessel becomes difficult. As a result, there are cases in which the starting property of the discharge lamp is degraded.
Generic technology is described in Japanese patent disclosure document 2002-100323. In this technology, a high pressure discharge lamp and an illumination device are described in which there is a UV radiation source as the starting aid. However, in the technology described in this document, mainly a high pressure discharge lamp or an illumination device is described, with the purpose of space illumination. It is used specifically in the situation in which few vibrations and the like are applied. Therefore, the attachment of the discharge vessel for starting is carried out, for example, only by a conductive body which is wound around the outside of the vessel comprising the UV radiation source. As a result, there is the disadvantage that, for an application in which the device is often moved, such as in a liquid crystal projector device, and in which high reliability with respect to the vibration resistance and impact strength is required, reliability is lacking. Since, in this technology, the distance between the starting aid-UV radiation source and the high pressure discharge lamp is relatively small, often heating from this discharge lamp takes place. Therefore, the process for attachment to a lamp by means of a cement or the like cannot be undertaken, for example.